Radiofrequency (RF) amplifiers are commonly used in numerous applications, such as video systems. Video signals which may be amplified by an RF amplifier have frequency characteristics, in which they include an RF (high-frequency) carrier, typically having a fundamental frequency of around 2 gigahertz (GHz), and a somewhat lower frequency component which modulates the carrier. This signal component is called the video signal. Depending on the application, the video bandwidth can be anything from 30 kilohertz (KHz), such as for cellular telephones, to 20 megahertz (MHZ), such as for W-CDMA applications. RF bandwidth generally includes the fundamental frequency and a first harmonic, double the frequency of the fundamental.
A Conventional RF Amplifier Architecture
A common-high-level architecture for an RF amplifier is shown in FIG. 1. For the sake of the present discussion, the conventional amplifier is depicted generally, or schematically, as an operational amplifier. However, it will be understood that the points made in connection with FIG. 1 have general applicability to RF amplifiers, regardless of their particular design.
An input signal 2, such as the video signal just described, is to be amplified. The elements of the RF amplifier will be explained below, in connection with a discussion of the characteristics of the RF amplifier. Typically, the RF amplifier is built using a printed circuit (PC) board, with the components installed on the board.
In general, desirable characteristics for amplifier circuits include linearity and impedances which have well-defined values.
Linearity is important because, the more linear the characteristic of the amplifier, the less signal distortion is introduced as a consequence of the amplification. Linearity is measured over a band of frequencies. In RF amplifiers, for instance, it is desirable that the linearity cover a band including both the carrier and the video signal.
There are techniques, well known in the field of amplifier design, for giving amplifiers linear characteristics. For instance, negative feedback, from the output of an active element back to an input, works well to produce linear characteristics. The amplifier circuit of FIG. 1 uses a differential-input operational amplifier (op-amp) element 4 as the active element. A negative feedback coupling 6 is provided between the output and the negative input of the op-amp 4.
Persons skilled in the art of amplifier design will be familiar with numerous alternative designs and components which may serve as the active element. A power transistor, for instance, may be used.
The impedance of a circuit, or of a circuit element, can have profound effects on the suitability of a circuit for its purpose.
High input impedance, for instance, is desirable because the input of the amplifier is coupled to a signal source which itself has an output impedance. The output impedance of the source, and the input impedance of the amplifier, form a voltage divider, in which the signal is dropped over the two impedances in proportion to their impedance values.
For maximum power transfer, i.e., to get the greatest amount of the signal into the amplifier, the amplifier's real input impedance R.sub.L must be equal to the real output impedance R.sub.S of the source. Therefore, amplifier designers strive to make the input impedance equal the source impedance.
Conversely, there are circuit elements for which the impedance should be low, or which should vary with frequency in specified ways. As will be discussed in detail below, certain components within RF amplifier circuits require impedances with specific values, which vary for different frequencies.
In particular, to avoid perturbations in the DC caused by the signal, the impedance at the video frequency should be carefully controlled.
For LDMOS circuit technology, low impedance on the drain DC feed is critical. For bipolar circuit technology, low impedance is critical for both base and collector feeds.
It is also common practice in amplifier design to provide a direct current (DC) bias voltage. The signal to be amplified is then treated as a perturbation above and below that bias voltage. Where the bias voltage is positioned near the middle of a voltage range of linear operation, good, low-distortion amplification is produced. Accordingly, FIG. 1 shows a bias circuit 8 coupled to the output of the op-amp 4 through a bias feed line 9.
Another issue relating to RF amplifier design stems from the relative physical dimensions of the circuit elements and connections, versus the wavelengths of the RF signals to be amplified. Because of the high frequency (and, consequently, the short wavelength) of the RF signals, there may be a significant phase difference in the signal between the ends of the bias feed 9. Circuit designers deal with this phase difference through the well-known theory of transmission lines.
An important issue in transmission line theory is that of impedance matching. Where there is an impedance mismatch between a signal source and a signal destination, the signal may be reflected back along the transmission line connection toward the source. This reflection leads to undesirable signal reflection loss. Impedance matching is employed to eliminate this undesirable reflection. Accordingly, in FIG. 1, an impedance matching circuit, shown schematically as 10, is provided between the output of the op-amp 4 and the output 12 of the overall RF amplifier circuit.
Therefore, a conventional RF amplifier will have the general architecture given in FIG. 1, generally having the active element 4, the bias 8, and the impedance matcher 10.
The Problem of Reconciling Bias Circuit Impedance Requirements
In RF amplifiers such as that of FIG. 1, there are two requirements, which tend to conflict with each other.
One requirement is that the bias circuit 8 should have a low impedance at the frequency of the signal modulation. That impedance is referred to as the video impedance. By keeping the video impedance low, linearity is maintained and distortion of the video signal is minimized.
The other requirement is that, at the RF carrier frequency, the bias circuit 8 should have a high impedance, to prevent loading of the matching circuit 10. The impedance of the bias circuit 8 should preferably be at least an order of magnitude higher than the impedance at the connection points.
It is possible to design RF amplifier circuits, and to configure their physical implementations, such that impedance is a function of the frequency of the signal in question. Thus, a suitably designed RF amplifier can have two different impedance values in the carrier frequency range and the video signal frequency range.
However, in general such circuit design becomes more difficult as bandwidth requirements, i.e., linearity requirements across bands, become greater, and as bands which have different requirements get closer together across the frequency spectrum. This problem has become increasingly difficult, because new technologies have caused the video bandwidth requirements to increase. For instance, while FM cellular technology has called for a video bandwidth of 30 kHz, newer W-CDMA applications require a video bandwidth of 20 MHZ Additionally, physical implementations, such as implementations on printed circuit boards, impose cost and space requirements which further limit the designers flexibility for achieving RF amplifiers of a given output power and linearity.
Prior Art Solutions
With the foregoing issues and criteria in mind, amplifier designers have tried several approaches to produce DC bias circuits so as to achieve the desired objectives.
1. Quarter-Wavelength Transmission Line
The most common bias circuit design, generally used in PC board implementations is to print a transmission line between the bias circuit 8 and the junction between the active element 4 and the matching network 10. The transmission line has high characteristic impedance compared to the matching network 10, and has a length which is a quarter wavelength of the RF carrier. At the end of the transmission line, a low RF and DC impedance is established, using filter capacitors.
This design has two drawbacks. First, the length of the transmission line is great enough to take up an undesirably large amount of PC board real estate. Second, it has been found that the impedance at 20 MHZ, resulting from the length of the transmission line tends to be too high for good linearity.
2. Discrete Inductor
Another design includes a discrete inductor, positioned between the bias circuit 8 and the matching circuit 10. This design also has the drawback that PC board real estate is taken up, this time by the inductor. Also, it has been found that, while the RF impedance is suitably high, the video impedance, around 20 MHZ, is undesirably high.
3. Parallel Tank Circuit (U.S. Pat. No. 5,272,450)
Yet another conventional design is given in Wisherd, U.S. Pat. No. 5,272,450, "DC Feed Network for Wideband RF Power Amplifier." In Wisherd, the DC bias circuit is coupled through a parallel tank circuit.
A capacitor is included, for resonating out the inductive loading at RF, thereby allowing a lower value inductor to be used. The lower value inductor is smaller, and takes up less board real estate. Also, the lower inductance value provides lower impedance at the video frequency.
However, this design also has several drawbacks. The two discrete components (the inductor and the capacitor) may introduce undesirable parasitic loading effects on the matching circuit. Also, lowering the inductance degrades the RF bandwidth and the circuit's Q increases. (Q is a numerical measure of the circuit's bandwidth. The value of Q increases as the bandwidth decreases.)
4. Shortening the Bias Feed
By contrast with the first conventional technique, the bias feed can be made shorter than a quarter wavelength. This technique reduces the video impedance, but has several drawbacks. First, the RF impedance at the fundamental frequency is also reduced. Additionally, the RF impedance varies rapidly over the RF bandwidth (e.g., between the 2 GHz fundamental and the 4 GHz first harmonic). In particular, the impedance at the second harmonic tends to be undesirably high.
Therefore, there is an unmet need for an approach to RF amplifier design which achieves the desired circuit characteristics given above, and which avoids consuming unnecessarily large amounts of PC board real estate.